This invention relates to methods and apparatus for telecommunication and in particular to packet-switched (PS) communication systems that are adapted to handle re-starts and re-allocation of system nodes.
In a packet data communication system, information is exchanged as packets of digital data, or datagrams. Each data packet includes address information that enables the system to direct each packet on its own way through the system from a sender to a receiver. Thus, a packet data communication system does not maintain a continuous connection between a sender and a receiver. Packet data communication systems are sometimes called “connection-less” and packet-switched systems, distinguishing them from traditional telephony systems in which continuous connections are established between senders and receivers. Thus, traditional telephony systems are sometimes called “connection-oriented” and circuit-switched (CS) systems.
General packet radio service (GPRS) is a packet-switched communication system that is standardized by the European Telecommunications Standards Institute (ETSI) and the Third Generation Partnership Project (3GPP). See for example “Digital Cellular Telecommunications System (Phase 2+) (GSM); General Packet Radio Service (GPRS); Service description; Stage 2”, 3GPP TS 03.60 ver. 7.6.0 Release 1998; and “General Packet Radio Service (GPRS); Service Description; Stage 2”, 3GPP TS 23.060 ver. 3.3.0 Release 1999 (Apr. 2000). GPRS is also described in H. Granbohm et al., “GPRS—General Packet Radio Service”, Ericsson Review No. 2, pp. 82-88 (1999) and in L. Ekeroth et al., “GPRS Support Nodes”, Ericsson Review No. 3, pp. 156-169 (2000).
GPRS operates with circuit-switched, cellular mobile telephone systems such as the Global System for Mobile (GSM) system, also standardized by ETSI and 3GPP, and the U.S. time division multiple access (TDMA) cellular system defined by the TIA/EIA-136 standard promulgated by the Telecommunications Industry Association (TIA) and Electronic Industries Association (EIA). By adding GPRS functionality to GSM and TDMA public land mobile networks (PLMNs), network operators can give their subscribers resource-efficient access to external Internet protocol-based (IP-based) packet data networks (PDNs) like the Internet.
As depicted in FIG. 1, a GSM-style PLMN includes a number of interconnected network nodes, in particular, a mobile switching center/visitor location register (MSC/VLR), a home location register (HLR), and base station systems (BSS). The BSS handles radio communication with subscribers' mobile stations (MSs) via an air interface Um. The HLR is a database of information about the subscribers that is accessed by the MSC/VLR via a D-interface and that is accessed by a serving GPRS support node (SGSN) via a Gr-interface. The MSC/VLR routes circuit-switched calls to and from the MSs, communicating with the BSS over an A-interface. It will be appreciated that these nodes are typical of a circuit-switched network such as a PLMN, whether GSM or not. Data transfer and signaling interfaces are indicated in FIG. 1 by solid lines and signaling interfaces are indicated by dashed lines.
Packet data services and GPRS add nodes in a packet-switched portion of the communication network for handling packet data traffic; these nodes interwork with the circuit-switched portion of the communication system depicted in FIG. 1. For example, an SGSN is connected to the BSS via a Gb-interface and resides at the same hierarchical level in the network as the MSC/VLR. A gateway GPRS support node (GGSN) is the interconnection point to a packet data network via a Gi-interface and is connected to the SGSN via a Gn-interface (which may be an IP backbone). User data to the Internet, directed for example, from a terminal equipment (TE) connected to a mobile terminal (MT), is sent encapsulated over the IP backbone. In FIG. 1, R is a reference point between a non-ISDN compatible TE and an MT. In this application, the end-user's equipment is called a mobile station (MS) whether it is a combination of a phone (MT) and a device such as a computer (TE) or just a phone.
The SGSN and GGSN can be combined into one physical node and deployed at a central point in the network, or a network may include several GGSNs and SGSNs as shown. Packet data streams and short text messages are handled in FIG. 1 by a Short Message Service—Gateway MSC (SMS-GMSC) and an SMS—Interworking MSC (SMS-IWMSC) that communicate with the HLR via a C-interface and with the MSC/VLR via an E-interface. As seen in FIG. 1, the SMS-GMSC and SMS-IWMSC exchange short messages with a short message switching center (SM-SC), and the SMS-GMSC communicates with the SGSN via a Gd-interface. It will be appreciated that the nodes depicted in FIG. 1 are typical of a packet-switched network, whether a GPRS network or not. It will also be appreciated that some networks physically split node(s) into control plane node(s) and user plane node(s) in order to enable independent scalability of signaling traffic and data traffic, among other reasons.
Most of the interfaces depicted in FIG. 1, and in particular the Gs- and A-interfaces, exchange messages with the help of the Signaling System Number 7 (SS7) that is standardized by ETSI and the American National Standards Institute (ANSI), among others. SS7 in GSM and GPRS uses a message transfer part (MTP) protocol to deliver messages and a signaling connection control part (SCCP) protocol for extended addressing. The SCCP protocol provides for each message to have an SCCP header that has a sub-system number for telling the node receiving the message which application should have the message. An SGSN, for example, typically has different sub-system numbers for communication with the HLR and with the MSC/VLR. An MSC usually derives the node type of a communicating peer node based on the sub-system number that may be stored in a database or included in an earlier message.
In a GPRS network, packet data channels (PDCHs) are mapped onto respective timeslots, thereby utilizing the same physical channel structure as ordinary circuit-switched GSM/TDMA channels. All radio resources are managed from a base station controller (BSC) in the BSS, which also includes Base Transceiver Stations (BTS); the pool of physical channels for a given cell can be used as either circuit-switched channels or packet-data channels. By packet multiplexing, the allocated PDCHs can be shared by every GPRS user in the cell, and the number of PDCHs in a cell can be fixed or dynamically allocated to meet fluctuating traffic demands. To support efficient multiplexing of packet traffic to and from mobile stations, or mobile terminals (MTs), packet data traffic channels (PDTCHs), packet associated control channels (PACCHs), and packet data common control channels (PDCCHs) are specified for the air interface Um, although PDCCHs are not always used.
As noted above, an SGSN serves every GPRS subscriber that is physically located within the SGSN's service area. To a large extent, the SGSN does for packet-switched service what the MSC/VLR does for circuit-switched service. The mobility management functions for GPRS terminals that are performed by an SGSN include attach/detach, user authentication, ciphering, location management, and so on, and an SGSN supports combined mobility management for at least some mobile terminals by interworking with the MSC/VLR. An SGSN also manages the logical link to mobile terminals that carries user packet traffic, SMS traffic, and layer-3 signaling between the network and the GPRS terminals. An SGSN also routes and transfers packets between mobile terminals and the GGSN; handles packet data protocol (PDP) contexts (the PDP context defines important parameters, such as the access point name, quality of service, the GGSN to be used, and so on, for connection to the external packet data network); interworks with the radio resource management in the BSS; and generates charging data.
As noted above, the GGSN accommodates the interface to external IP-based networks. Access-server functionality in the GGSN is defined according to standards from the Internet Engineering Task Force (IETF). The GGSN functions as a border gateway between the PLMN and external networks, sets up communication with external packet data networks, authenticates users to external packet networks, routes and tunnels packets to and from the SGSN, and generates charging data.
The MSC/VLR also supports integrated mobility management for mobile terminals. GPRS attach and PDP-context activation must be executed in order for GPRS users to connect to external packet data networks. The mobile terminal makes itself known to the network by means of GPRS attach, which corresponds to IMSI attach used for circuit-switched traffic. Once the terminal is attached to the network, the network knows its location and capabilities. For some mobile terminals, circuit-switched IMSI attach and packet-switched GPRS attach can be performed at the same time.
GPRS attach is depicted by FIG. 2. In step 1, the mobile terminal requests that it be attached to the network. The terminal's request, which is sent to the SGSN, includes parameters that indicate its multi-timeslot capabilities, the ciphering algorithms it supports, whether it wants to attach to a packet-switched service or to both packet- and circuit-switched services, etc. In step 2, authentication is made between the terminal and SGSN, which may fetch relevant data from the HLR. In step 3, subscriber data from the HLR is inserted into the SGSN; and in step 4, information is passed to the terminal that indicates the terminal is attached to the network.
Before the mobile terminal can communicate with an external PDN (e.g., an IP network), a PDP context must be activated. The PDP context includes parameters that describe the characteristics of the connection to the external PDN, e.g., the address allocated to the MS, access point name (APN), quality of service (QoS), and so on. PDP contexts may be primary or secondary, in which a secondary PDP context uses the same MS IP address and is connected towards the same APN (i.e., external net) as its respective primary PDP context. A composite PDP context contains one primary and zero or more secondary PDP contexts.
PDP-context activation is depicted in FIG. 3. In step 1, the mobile terminal requests PDP-context activation. In step 2, the SGSN validates the request based on subscription information received from the HLR during GPRS attach. In step 3, the APN is sent to a domain name server (DNS) from the SGSN to find the IP address of the relevant GGSN. In step 4, a logical connection is created between the SGSN and the GGSN (i.e., a GPRS Tunneling Protocol (GTP) tunnel is formed). In step 5, the GGSN assigns a dynamic IP address to the mobile terminal, if required, from the range of IP addresses allocated to the PLMN or externally, from a Remote Authentication Dial-In User Service (RADIUS) server (a fixed IP address from the HLR could also be used). A RADIUS client is included in the GGSN to support Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP) authentication to external networks with RADIUS servers. After an acknowledgment of the PDP context activation is returned to the MS (step 6), communication between the user and the external PDN (e.g., an Internet Service Provider (ISP) network or a corporate network) can commence (step 7).
In GPRS, establishing a PDP context signifies establishing a communication session in an MS, the radio network, an SGSN, and a GGSN. In addition, GPRS includes a “heartbeat” mechanism implemented by Echo Request and Echo Response messages that every node/entity sends to all of its communication peers over the Gn- and Gp-interfaces, and if a reply is received and a re-start counter of the peer is different from the previously received counter value, the peer is considered re-started.